1. Field of the Invention
The present invention relates to a semiconductor manufacturing process. More particularly, the present invention relates to a method of planarizing a spin-on material layer and manufacturing a photoresist layer.
2. Description of the Related Art
With an increasing level of circuit integration, the size of the circuit devices continues to shrink. Photolithographic processing plays a very important role in the fabrication of semiconductor devices. All metal-oxide-semiconductor (MOS) related structures including various types of thin film patterns and doped regions are defined by performing photolithographic processes. In fact, the degree of device integration in the semiconductor industry ultimately depends on the production of features with a narrower width in a photolithographic process.
Photolithography is a process that includes a few steps. First, a layer of exposure material called photoresist is formed on the surface of a wafer. Thereafter, light from a planar light source is permitted to illuminate the photoresist layer after passing a photomask having a pattern thereon so that the photoresist layer is selectively exposed. Hence, the pattern on the photomask is transferred to the photoresist layer.
The aforementioned step of depositing a photoresist layer over a wafer is one of the most frequently performed processes in the fabrication of semiconductor. FIGS. 1A and 1B are schematic cross-sectional views showing the step height after coating a photoresist layer over a wafer having different density of openings across its surface. As shown in FIG. 1A, the substrate 100 is a silicon substrate or a semiconductor structure including a dielectric layer, a semiconductor material layer or a conductive layer. The substrate 100 has a plurality of openings 102. These openings 102 are spread out across the surface of the substrate 100 with different degrees of concentration to produce a dense opening region 104 and a sparse opening region 106. In general, the dense opening region 104 refers to an area on the substrate 100 having two or more openings 102 close to each other. On the other hand, the sparse opening region 106 refers to an area on the substrate having a single opening or two openings separated from each other by a large distance.
As shown in FIG. 1B, a photoresist layer 108 is formed over the substrate 100 by spin coating. The photoresist layer 108 completely fills the openings 102. Because of the difference in the density of openings between the dense opening region 104 and the sparse opening region 106, the thickness of the photoresist layer 108 above the dense opening region 104 is different from the sparse opening region 106. Hence, a step height 110 is produced between the two regions 104, 106.
The aforementioned difference in step height of the photoresist layer 108 over the substrate 100 often leads to problems such as non-uniform etching or inaccurate focusing. To reduce the step height, a method that involves repeated etching back process and photoresist coating process (for example, in U.S. Pat. No. 6,482,716 and U.S. Pat. No. 6,630,397) is deployed. In other words, the substrate is transferred to an etching station to perform an etching back operation immediately after the step height is produced. Thereafter, the substrate is transferred to a photolithographic station to perform another photoresist coating process. This procedure is repeated several times so that the step height is gradually removed. However, the aforementioned method can at most reduce the step height but cannot eliminate the step height altogether. Furthermore, the repetitive etching in the etching station followed by the coating in the photolithographic station not only wastes processing time, but also increases the production cost considerably.
In addition, the formation of a step height is not limited to the process of coating a photoresist layer. Similar phenomenon may also occur in spin-coating other materials, such as the bottom anti-reflection coating and the spin-coated glass over a substrate. In other words, a method capable of resolving the step height problem will also resolve a host of similar problems.